1. Field of the Invention
The present invention relates to cationic starch and a cationic starch paste or slurry for papermaking which has a high nitrogen content and yet has a low cation equivalent value. The present invention relates also to a process for producing the same.
Major terms and abbreviations used in this specification are defined below.
(1) Cation equivalent value (meq): Expressed in terms of values measured by the colloidal titration of starch paste solution with PVSK (potassium polyvinylsulfate).
(2) Viscosity: Expressed in terms of values measured on a 1 wt % sample at 50.degree. C. with a Brookfield viscometer, unless otherwise mentioned.
(3) Nitrogen content: Expressed in terms of percentage (%) values measured on a sample of cationic starch by the semimicro Kjeldahl method.
(4) DS value: Expressed in terms of an average of the number of substituted hydroxyl groups per glucose residue. It represents the degree of esterification and etherification of a derivative.
2. Description of the Prior Art
The recent trend in the papermaking industry is toward the use of increasing amounts and varieties of cationic chemicals for stable papermaking machine operation under the condition that more DIP (deinked pulp) and waste paper pulp are used than before. The consequence is an overall increase of cation equivalent value in the papermaking system containing as much general-purpose cationic starch as before. This in turn raises the zeta potential in the system, making it difficult to maintain it in the optimum range of -5 mV to .+-.0 mV in the wet end of paper making system. Therefore, although Cationic starch is essential for dry strength, it is necessary to limit cationic starch in its dosage amount and its degree of cationization. Such limitation, however, decreases the effect of cationic starch on flocculation or bonding of pulp fiber or fine fiber which is proportional to the amount and the degree of cationization.
The degree of cationization is proportional to nitrogen content in cationic starch, because cationic starch is produced by replacing the hydroxyl group in starch with a quaternary ammonium salt and/or a tertiary amino group. Thus, there has been a demand for a cationic starch and a cationic starch paste solution thereof for papermaking which has a high nitrogen content (which relates to the degree of cationization and the amount of cationic starch) and yet has a relatively low cation equivalent value.
It is common practice to use cationic starch powder in the papermaking process by adding it in the form of a starch paste or slurry to the machine chest, mixing chest, or fan pump for uniform dispersion into the paper stock. Uniform dispersion is desirable for improved paper strength and improved size yield.
Unfortunately, these requirements are not met by conventional cationic starch which has a high solution viscosity because it doe snot undergo acid treatment or oxidation (to lower the viscosity) in its manufacturing process. Conversely, lowering the viscosity also reduces the molecular weight of cationic starch, which adversely affects the performance of cationic starch. This is another reason why there has been a demand for a cationic starch and starch paste solution thereof for papermaking that has the above-mentioned dual characteristic properties.